In process and automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are measuring devices, such as, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH-measuring devices, conductivity measuring devices, etc., which register corresponding process variables, fill level, flow, pressure, temperature, pH value, and conductivity, respectively. Used for influencing process variables are actuators, such as valves or pumps, via which e.g. the flow of a liquid in a pipeline or the fill level of a medium in a container can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. A large number of such field devices are available from members of the firm, Endress+Hauser. In connection with the invention, the terminology, field device, thus includes all types of measuring devices and actuators. Furthermore, the terminology, field device includes also e.g. a gateway, a radio adapter or other bus participants integrated/integrable in a bus system.
Rough conditions of use reign in many industrial plants, so that field devices can experience very high loads. During the planning of an industrial plant, the loadability of the field devices, which are to be applied, should be carefully considered. Main concerns, in such case, are not only the durability of the field devices per se, but, instead, also the reliability and the functional ability of the measurements technology integrated in the field devices. Themes such as lifetime, self monitoring and predictive maintenance play an important role here. These considerations hold especially for measuring devices applied for registering process variables.
Vortex flow measuring devices are applied, for example, in order to register the flow velocity of a medium in a measuring tube. In order to prevent negative outcomes, the measurements technology applied in vortex, flow measuring devices must deliver reliable measured values and be failsafe. In plants, in which actuators influence the same or another process variable based on process variables registered by measuring devices, considerable damage can result, when the actuators work based on unreliable measured values. In this sense, vortex flow measuring devices must not only function, but also work reliably, especially also in plants, such as, for example, a steam power plant, in which the lines are partially high pressure lines, especially such lines containing media heated to high temperatures.
The example of a vortex, flow measuring device will be developed further in the following. Vortex flow measuring devices are in the normal case composed of a measuring tube having a bluff body arranged in the measuring tube. Vortices form in media flowing past this bluff body, provided that the media, respectively the flows of the media, fulfill certain conditions.
Playing an important role in this connection is the Reynolds number. The Reynolds number is a dimensionless variable, which characterizes the ratio between inertial forces and viscosity forces of a flow system. When the Reynolds number of a system lies in a certain region, vortices are formed in the medium flowing past the bluff body in the measuring tube. The vortices are such that they form a so-called Kármán vortex street. The vortices are shed alternately from the surface of a first side of the bluff body and from the surface of a second side of the bluff body. The vortices have different rotational directions as a function of the side on which they are shed. The vortices are always shed with equal separation from one another. This means that the vortex shedding frequency is influenced only by a change of the flow velocity of the flowing medium. The vortex shedding frequency is thus directly dependent on the flow velocity.
Measuring devices, which register the vortex shedding frequency, have a sensor system, respectively measurements technology, which registers the pressure fluctuation as a result of vortex formation. Known is a sensor system, which has a capacitive sensor and a paddle. The paddle protrudes into the measuring tube and is moved from one side to the other under the influence of the vortices. The lateral movements are registered via a capacitance change. At least one capacitor is arranged in a chamber for registering the capacitance changes, wherein the chamber is sealed from the process. The movement of the paddle is transferred to a movable element within this chamber via a membrane, or diaphragm. The membrane serves for sealing the space from the process as well as for recording the process pressure.
Measuring devices for registering process variables are frequently equipped with seals. The seals serve to block from the electrical, electronic or also mechanical components of the measuring device moisture originating from the process and/or the environment. If a seal is defective, moisture can strongly influence the ability of the measuring device to function. For example, a membrane in a vortex flow measuring device can in the case of high loading break open, or small cracks can form in the membrane. Thus, in an automated plant, the case can occur that a valve in the plant opens or closes too rapidly. As a result of thereof, a pressure wave forms in a pipeline, which can cause considerable damage. Among other things, the membrane of a vortex flow measuring device can be damaged, wherein the damage can involve small openings/cracks, so that medium can penetrate therethrough into the interior. The penetrating moisture can cause short circuits in the electrical and/or electronic components, or over longer periods of time result in corrosion.
Leakage electrical current, which flows between a measuring device electronics and a grounded measuring device housing, can be monitored, in order to detect a defective operating state of a measuring device. For example, known from Published International Application WO 2009/135764 A1 is a method for monitoring a measuring device, especially a measuring device formed as a measuring- and/or switch device of industrial measuring- and automation technology and/or an electronic measuring device. In the case of the known solution, a potential difference is produced between the housing and the measuring device electronics for effecting a leakage current, wherein the leakage current flows through an electrically conductive connection between the housing and the measuring device electronics formed by condensed water. Furthermore, the flowing leakage current is registered, and a digital state value is generated taking into consideration the registered leakage current. Then, an alarm signal is generated, which signals the occurrence of a failure in the measuring device—especially a failure caused by the undesired formation of conductive deposits within the housing.
Disadvantageous in the case of the known method is that an electrical current measuring circuit is required for registering the leakage current. It is further in this method only measured whether instantaneously an electrically conductive connection is present or not. The method registers a failure only in the case of a leakage current flowing to ground. Predictive maintenance is not possible with the known solution.
Known from US 2013/0207677 A1 is a method for detecting a malfunction of an electrostatic, capacitive sensor, wherein the method includes steps as follows:                detecting a first detection signal as a function of an electrostatic capacitance, which is ascertained via a detection electrode, when a first electrical voltage is placed on a shield electrode, which is provided in the vicinity of the detection electrode;        detecting a second detection signal as a function of an electrostatic capacitance, which is ascertained via the detection electrode, when a second electrical voltage different from the first voltage is placed on the shield electrode; and        determining due to the first acertainment signal and the second acertainment signal, whether a disturbance of the detection electrode is present.        
A disadvantage of the above described solution is that an additional detection electrode is provided. Additionally, at least two different reference voltages are required.